Neurodevelopmental disorders are common and represent a significant burden to the well-being of patients and their families. A greater understanding of neuronal development in both the normal and disease states will contribute to general biological knowledge and may also lead to preventions or cures for such disorders. The mouse visual system is a particularly useful model system to study such phenomena as it has been extensively studied on the molecular, cellular, and physiological levels and the principles learned by studying retinal neurons are likely to be generally applicable to neurons in the brain. Recent studies from the sponsor's laboratory have revealed a role for the Down syndrome cell adhesion molecule (Dscam) and the related Dscaml1 genes (hereafter collectively referred to as Dscams) in self-avoidance of neurons within the mouse retina. As its name suggests, Dscam is found within the critical interval of human chromosome 21 implicated in Down syndrome. In the absence of DSCAM or DSCAML1, cell type-specific aggregation and dendrite fasciculation is seen in the subsets of retinal neurons that would normally express the protein. In the wild-type eye, these neurons are evenly spaced and their neurites rarely self-cross. It has long been assumed that the spacing and morphology of retinal neurons is important for the faithful processing of visual information, and Dscam mutant mice represent the first appropriate system to directly test this hypothesis. In Specific Aim 1, I will use established behavioral tests of bot image-forming and non-image-forming vision to assess the functional consequences of the altered anatomy caused by mutations in Dscams. Since functional synapses still form in the mutant and non-image-forming visual tasks are less likely to depend on a topographical map, my hypotheses are as follows: 1) image-forming vision will be negatively affected in cases where the cell type(s) necessary for the given behavior have been disrupted by loss of Dscams, and 2) non-image-forming vision will remain intact. Preliminary data from tests of optomotor and pupillary responses support these hypotheses. In Specific Aim 2, I will endeavor to understand the molecular mechanisms of DSCAM function and to identify additional proteins important for neuronal morphogenesis. This will be accomplished by exploiting the known association of the C-terminus of DSCAMs with one of the six PDZ domains of MAGIs (membrane-associated guanylate kinases with inverted domain structure). I will immunoprecipitate DSCAM/MAGI complexes and identify additional protein components via mass spectrometry. I hypothesize that the proteins identified will fall into two categories: signal transduction proteins that mediate intracellular signaling of DSCAM, and proteins similar to DSCAM that simultaneously bind to other PDZ domains of MAGIs and which are also involved in communicating information regarding cell type. This multi-level approach will be an effective means of elucidating both the pathogenesis of neurodevelopmental disorders such as Down syndrome and general neurobiological principles of development.